US10197470B2 - Hydrocarbon leak imaging and quantification sensor - Google Patents

Hydrocarbon leak imaging and quantification sensor Download PDF

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US10197470B2
US10197470B2 US15/598,052 US201715598052A US10197470B2 US 10197470 B2 US10197470 B2 US 10197470B2 US 201715598052 A US201715598052 A US 201715598052A US 10197470 B2 US10197470 B2 US 10197470B2
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US20170336281A1 (en
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Allen M. Waxman
Jason M. Bylsma
Allan Vaitses
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Multisensor Scientific Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
    • G01M3/00—Investigating fluid-tightness of structures
    • G01M3/38—Investigating fluid-tightness of structures by using light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/359—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using near infrared light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/85—Investigating moving fluids or granular solids
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0027—General constructional details of gas analysers, e.g. portable test equipment concerning the detector
    • G01N33/0036—General constructional details of gas analysers, e.g. portable test equipment concerning the detector specially adapted to detect a particular component
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/66—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
    • G01F1/661—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters using light
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/2823—Imaging spectrometer
    • G01J2003/2826—Multispectral imaging, e.g. filter imaging
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G01N2021/3513—Open path with an instrumental source
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G01N2021/3531—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis without instrumental source, i.e. radiometric
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/85—Investigating moving fluids or granular solids
    • G01N2021/8578—Gaseous flow
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02A—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE
    • Y02A50/00—TECHNOLOGIES FOR ADAPTATION TO CLIMATE CHANGE in human health protection, e.g. against extreme weather
    • Y02A50/20—Air quality improvement or preservation, e.g. vehicle emission control or emission reduction by using catalytic converters

Definitions

  • This invention refers generally to optical detection and quantification of natural gas and other hydrocarbon gas leaks, from holes and cracks in pressurized vessels, pipes, components, and general gas infrastructure, and from emissions emanating from surfaces due to gas leaks in underground gas infrastructure or naturally occurring surface emissions. It may also be useful in assessing methane emissions from livestock.
  • Natural gas leaks create both safety and environmental hazards, and occur along the entire gas supply chain from the well to the street (so-called upstream, midstream, and downstream sectors).
  • Methane the primary constituent of natural gas is combustible in air, and is also a potent greenhouse gas.
  • Other hydrocarbons found in natural gas, as well vapors emanating from liquids separated from gas and oil include ethane, propane, butane, pentane, hexane, octane, and heavier hydrocarbons, which form volatile organic compounds that generate smog which is a health hazard.
  • Gas detectors can be classified according to their coverage extent, as either spot sensors, line sensors or area sensors.
  • Spot sensors often referred to as sniffers, draw in a local sample of air and detect the presence of a combustible or toxic gas by means of various analytical methods. They can be fixed in place for continuous monitoring, or hand portable for inspections, but they require direct sampling in place and provide very limited coverage. They may provide concentration measurements, but do not provide leak rate estimates. Other instrumentation is available to locally sample (as opposed to image) known leaks in order to provide an estimate of leak rate, but they too provide only local coverage and require direct collection of gas from the leaking component.
  • Optical line sensors also known as open-path gas detectors, employ optical means to detect gas that lies along the line between a dedicated light emitter (e.g., laser, tunable laser, or narrowly focused broadband source) and a dedicated photo-detector (or multiple photo-detectors).
  • a dedicated light emitter e.g., laser, tunable laser, or narrowly focused broadband source
  • a dedicated photo-detector or multiple photo-detectors
  • These sensors detect gas present anywhere along the line between the light emitter and the photo-detector (or between combined emitter/detector assembly and a remote reflector if the optical path is folded), but they cannot determine where along the path the gas is, nor from where it came, and has limited coverage to only the narrow open path between emitter and detector.
  • Such sensors can measure column density of gas along the open path, but cannot measure or estimate concentration nor leak rate.
  • Open-path sensors can be installed in place, hand portable, or mobile aboard ground and air vehicles. In order to achieve area coverage from a standoff distance, it is recognized that imaging sensors offer many advantages over spot and line sensors, in that they can detect the presence of gas and possibly localize the leak source.
  • gas imaging technologies have been proposed, developed, patented, and are commercially available. They are all based on the absorption of infrared light at wavelengths characteristic of the molecules of interest. For methane and hydrocarbons in general, most imagers operate in select bands of the mid-wave infrared and long-wave infrared spectrum. The leading commercially available gas imaging sensors operate in only a single narrow band of the mid-wave infrared spectrum, and do not provide quantitative data, only pictures to be interpreted by the human operator. Other imaging sensors utilize multiple spectral bands in the long-wave infrared (the so-called “molecular fingerprint region”) to detect and discriminate among different hydrocarbon gases, and to quantify the column density of gas at each pixel of the image.
  • the so-called “molecular fingerprint region” multiple spectral bands in the long-wave infrared
  • thermo-electric cooling to reduce dark current in gas imaging sensors.
  • none of the available gas imaging sensors provides a capability to estimate leak rate from a hole, or emission flux from a surface. Some can provide column density of gas at each pixel, and using spatial information of the imaged gas jet, plume or cloud, one can then estimate local or average gas concentration.
  • Multiple embodiments of the invention are described and have been developed, that are applicable more generally to natural gas and other hydrocarbon gases, liquids, emulsions, solids, and particulates, and to emissions monitoring of greenhouse gases such as methane and carbon dioxide.
  • This invention consists of sensors and algorithms for imaging, detection, localization, and quantification of hydrocarbon leaks by means of multispectral sensing using non-thermal infrared radiation from natural sunlight or artificial illumination sources. More specifically, several embodiments of sensor systems are described that incorporate short-wave infrared (SWIR) detector arrays sensitive in the range of approximately 1.0 through 2.6 microns, in combination with two or more spectral filters selected to create Core and Wings spectral bands with respect to a hydrocarbon feature complex in the vicinity of 2.3 microns. Detection is accomplished via absorption spectroscopy using natural sunlight or artificial illumination in direct transmission through a gas to the sensor, or reflected off a background surface with gas located between the background and the sensor. With the system properly calibrated, the resulting multispectral data can be processed in real-time to yield an absorption map or image related to the differential optical depth, or equivalently column density, of an intervening hydrocarbon gas such as methane, the major constituent of natural gas.
  • SWIR short-wave infrared
  • the resulting absorption imagery is color mapped to render the degree of gas absorption across the scene, and overlaid on an optically registered color visible image that provides context.
  • the escaping gas forms a turbulent jet or plume that is visible in the absorption image and from which the leak can be localized.
  • the invented methods estimate both the diameter of the effective hole and the mass flux of leaking methane (or other gas) from the data present in this absorption image, if the internal pressure driving the leak is known approximately.
  • underground gas leaks such as due to municipal gas infrastructure or gathering lines from gas wells, the gas percolates through the subsurface soil and emerges at the surface, often in disconnected surface patches.
  • the invented methods estimate both the mass of gas and the mass flux from a surface patch by combining the absorption imagery with wind speed and direction measured near ground level. Flux estimation methods are developed for cases of both steady winds and gusting winds.
  • This invention has several key advantages over thermal infrared gas imaging sensors that operate in the mid-wave (MWIR) or long-wave (LWIR) infrared parts of the spectrum. This includes the ability to detect and quantify leaked gas with small or no temperature difference relative to the background, as the invention utilizes SWIR light provided by natural sunlight or by lamps of appropriate color temperature, and does not rely on a thermal contrast between gas and the background or a background of varying temperature.
  • the detectors suitable for use in this invention do not require cryogenic cooling, using instead thermo-electric cooling that is more reliable and less expensive than cryogenic coolers such as a Stirling engine or liquid nitrogen.
  • the invention can also detect gas leaks in the presence of humidity, steam, fog, and light rain, as the hydrocarbon features detected in the SWIR do not overlap spectral regions where water vapor absorption is significant, which is important as one cannot control the presence of water vapor or fog in the atmosphere between the sensor and the leak source, and many industrial processes purposely mix steam with hydrocarbon gases.
  • This invention and its various embodiments will be useful in imaging, detecting, localizing, and quantifying natural gas leaks from components along the entire gas supply chain, from the well head to compressors to transmission pipelines to gate stations and underground distribution networks. Detection and quantification of volatile organic compounds (VOCs) in or near refineries, petrochemical plants, hydrocarbon storage tanks, or other industrial and commercial facilities will be possible. Landfill methane emissions mapping will be possible using this invention in combination with tomographic imaging around the periphery of a landfill. Similar tomographic three-dimensional mapping of gas over a refinery is possible, utilizing an airborne variant of this invention. This invention has also been shown to be capable of detecting liquid oil spills on land, sand, seawater, and sea ice.
  • inventions of the invention will prove useful in detecting and mapping oil films and emulsions at sea, oil spills in arctic waters, tar balls on beach sand, and damage to wetlands from oil spills.
  • the embodiments of the invention described herein are suitable for packaging in the form of, for example, hand-portable imaging sensors, ground vehicle-mounted inspection systems, vessel-mounted sensing systems, airborne surveying systems, relocatable trailer-mounted and fixed-site monitoring systems.
  • FIG. 1A illustrates the physical principles that underlie multispectral absorption imaging for natural gas detection.
  • FIG. 1B illustrates the methane spectrum in the infrared region from 1.5 to 10 microns, its primary spectral absorption features in the short-wave, mid-wave, and long-wave infrared regions, and the ratio of these methane absorption features to the corresponding water-vapor absorption features.
  • FIG. 2A illustrates the short-wave infrared spectra of the primary constituents of natural gas—methane, ethane, propane, butane, and carbon dioxide, as well as the spectrum of water vapor.
  • FIG. 2B illustrates the detailed methane spectral features in the range 2.1 to 2.6 microns, and its decomposition into a Core Band and Wings Band, along with the value of the average absorption cross-sections for this choice of Core and Wings Bands.
  • FIG. 3 shows a prototype user interface for a gas imaging sensor implemented on a touch-screen tablet, displaying a methane gas jet exiting a 1.5 mm orifice at 130 psig.
  • FIG. 4A illustrates an example of real-time video imaging of natural gas exiting a 10 mm orifice from a pipe pressurized at a low levels of only 1 ⁇ 4 psig in a mild crosswind.
  • FIG. 4B illustrates an example of real-time video imaging of methane jets emanating from a loosened hammer union pressurized to 500 psig in a 9 kph crosswind.
  • FIG. 4C illustrates an example of real-time imaging of ground surface emissions due to an underground natural gas pipe leak beneath a street in the Boston area.
  • FIG. 5A is a one-dimensional photo-detector array with its read-out circuitry, together with a pair of spectral filters that overlays the detector array and alternates between each of the two filters covering the detector array.
  • FIG. 5B is a pair of one-dimensional photo-detector arrays, each with its own read-out circuitry, and each with a different spectral filter positioned over it.
  • FIG. 6A is a two-dimensional photo-detector array and its read-out circuitry, with four different spectral filters, each filter overlaying one or more rows of detectors.
  • FIG. 6B is an array of four discrete photo-detectors with their individual read-out circuits, each detector covered with a separate spectral filter island.
  • the spectral filters form a spectral filter mosaic.
  • FIG. 7A is a system diagram of the video leak imaging and quantification sensor system.
  • FIG. 7B is a system diagram of the scan leak imaging and quantification sensor system.
  • FIG. 8A diagrams the imaging geometry for leak detection with sunlight ahead of the leak in direct transmission, passing once through a gas jet towards the sensor.
  • FIG. 8B diagrams the imaging geometry for leak detection with a source of artificial illumination from behind the leak (near the sensor), reflecting off a background material, passing twice through a gas jet and then to the sensor.
  • FIG. 9A shows a real-time absorption image of a methane gas jet exiting a 1 mm orifice from a test manifold pressurized to 1300 psig.
  • FIG. 9B shows three profiles of differential optical depth across the methane gas jet of FIG. 9A , corresponding to the pixel values sampled along the lines labeled a, b, and c.
  • FIG. 10A shows a graph of the estimated jet width along the axis of the methane jet of FIG. 9A , and a least-squares linear regression to these data points.
  • FIG. 10B shows a graph of the integrated differential optical depth across the width of the jet, along the axis of the methane jet of FIG. 9A , and a least-squares linear regression to these data points.
  • FIG. 10C shows a graph of the ratio of integrated differential optical depth to estimated jet width (i.e., the average differential optical depth) along the axis of the methane jet of FIG. 9A , and a least-squares linear regression to these data points.
  • FIG. 11 shows a graph of the integrated differential optical depth across the width of a methane jet exiting a narrow slit orifice of width 50 microns and length 1 cm, and various least-squares regressions to these data points.
  • FIG. 12A illustrates for a set of experiments, a graph of the intercept value of average differential optical depth normalized by orifice diameter vs. the internal pressure driving a methane jet from orifices of 1 mm and 0.7 mm, and compares the data to a smooth power-law curve.
  • FIG. 12B illustrates data from an extensive set of experiments of methane exiting round and slit orifices of various sizes across a large range of pressures.
  • the graph shows the measured mass flux per unit area of orifice vs. the internal pressure driving the methane jet, and a least-squares linear regression to these data points.
  • FIG. 13A illustrates an outdoor test setup used for imaging and estimating leak rate (mass outflow) of methane exiting round orifices under pressure in a crosswind in sunlight, in which the mass flowing into the release manifold is measured.
  • FIG. 13B shows a graph of estimated mass outflow compared to the measured mass inflow for twelve experiments using the test setup shown in FIG. 13A .
  • FIG. 14A illustrates (side view) geometry of an elevated LIQS sensor imaging ground surface gas emission in the presence of ground-level winds.
  • FIG. 14B illustrates (plan view) geometry of gas emission from a surface patch in the presence of ground-level winds.
  • This invention detects gas leaks via differential absorption imaging spectroscopy in the range 1.0 to 2.6 microns, exploiting spectral features of hydrocarbons in the short-wave infrared (SWIR) region, primarily in the wavelength range of 2.0 to 2.5 microns. These wavelengths are not typically associated with those in the thermal emission regions of the mid-wave infrared (MWIR) and long-wave infrared (LWIR) for objects at terrestrial temperatures. Appreciable thermal emission at around 2.0 microns requires objects at temperatures of around 1000° C. Instead, this invention relies on illumination sources like natural sunlight and lamps of color temperature near 1000° C. Thus, the invention can detect hydrocarbons at the same temperatures as their backgrounds by using external illumination instead of thermally emitted light.
  • SWIR short-wave infrared
  • FIG. 1A The principals underlying non-thermal infrared multispectral imaging of a gas leak are shown in FIG. 1A .
  • SWIR radiation from the sun or broadband artificial illumination directly or in reflection off background objects, transmits through the ambient atmosphere, passes through a gas jet or plume emanating from a source such as for example a leak, continues towards the sensor where it is filtered into multiple spectral bands and detected on a photo-detector array that is sensitive to SWIR photons.
  • Both the atmosphere and the gas absorb some of the light at wavelengths characteristic of the materials that comprise these media.
  • the primary absorber is methane
  • the primary absorbers are water vapor and other ambient gases that may include methane as well as carbon dioxide.
  • Incident light is also scattered out of the transmission path by particulates in the atmosphere and the gas leak itself. Light that is absorbed by the gas is subsequently reemitted in all directions, resulting in a reduction of light at characteristic wavelengths that is transmitted in the direction from the light source towards the sensor.
  • 1B illustrates this by plotting the methane absorption cross-section, and the ratio of water vapor to methane absorption cross-sections in narrow spectral bands where methane possesses strong spectral features, shown here on semi-logarithmic scales for wavelengths from 1.5 to 10 microns. It is clear that, despite the relatively weaker absorption cross-section for methane in the SWIR compared to the MWIR and LWIR, it has significantly higher absorption ratio to water vapor in the SWIR. Thus, for imaging gas in the presence of humidity or fog or steam, the SWIR region has particular advantage over both the MWIR and LWIR spectral regions. For many applications, this is an advantage, despite the lower absorption cross-section in the SWIR.
  • FIG. 2A shows a plot of absorption cross-section (on a linear scale) in the SWIR spectrum from 1.8 to 2.6 microns, for the various constituents comprising natural gas: methane, ethane, propane, butane, and carbon dioxide, as well as for water vapor.
  • the hydrocarbons possess broad feature complexes from 2.2 to 2.5 microns with much overlap in the range of 2.2 to 2.4 microns.
  • Methane can be separated from the other hydrocarbons by its reduced absorption in the 2.4 to 2.5 micron range.
  • the constituents of natural gas have spectral features in the SWIR that lie between the strong water vapor features below 2.0 microns and above 2.5 microns.
  • similar absorption features are present in the SWIR for liquid crude oil, oil-water emulsions, asphalt and tar.
  • the invention utilizes a minimum of two spectral bands; one called the Core Band which spans the spectral feature complex from approximately 2.25 to 2.45 microns (200 nm bandwidth), and the other called the Wings Band (serving as a reference band) which spans an interval of approximately 100 nm to either side of the Core Band.
  • spectral intervals are shown as the rectangular boxes in FIG. 2A .
  • the average absorption cross-section across the Core and Wings Bands are plotted over the methane spectrum in FIG. 2B .
  • the presence of methane can be both detected and quantified in terms of column density of methane.
  • other SWIR spectral bands can be selected to preferentially detect and quantify the other constituents of natural gas shown in FIG. 2A and related volatile organic compounds of interest in gas and oil production.
  • the exact location and extent of any of these bands is not critical to enabling a functional sensor, as long as they span regions both on and off the strong spectral features of the gas of interest.
  • the invention described here has been reduced to practice by building functional prototypes of a multispectral video imager and a scan imager for methane imaging, detection and quantification
  • the prototype dual-band video sensor images at 20 frames per second and displays gas absorption imagery overlaid on color visible imagery of the scene on a touch-screen user display.
  • the prototype system is hand-portable and interfaces to external networks via both wireless and wired interfaces.
  • the prototype 6-band scan sensor creates imagery of gas over a programmable and variable field-of-regard, by combining raster scanning with super-resolution image processing. The flexibility of switching among a variety of scan patterns enables this sensor to support both gas safety applications and emissions monitoring applications, in a cost-effective manner.
  • This scan imager is suitable for mast-mounting to overlook wide-area installations, using a programmable pan-tilt unit to effect scanning.
  • An alternative embodiment replaces the pan-tilt unit with scanning mirrors or a combination of scanning mirror and rotating optics, to enable compact packaging for a hand-portable gas imaging and quantification camera.
  • FIG. 3 illustrates a prototype graphical user interface for the video gas imager, showing touch-screen controls of the sensor and displaying an image of natural gas emanating from a 1.5 mm round orifice at a pressure of 130 psig (pounds per square inch gauge), taken outdoors in sunlight.
  • the color rendering of the gas jet absorption corresponds to pixel-level differential optical depth between the Core and Wings Bands, which can be converted to column density of methane and expressed in a variety of common units (molecules/cm 2 , % LEL-meters, ppm-meters).
  • FIG. 3 illustrates a prototype graphical user interface for the video gas imager, showing touch-screen controls of the sensor and displaying an image of natural gas emanating from a 1.5 mm round orifice at a pressure of 130 psig (pounds per square inch gauge), taken outdoors in sunlight.
  • the color rendering of the gas jet absorption corresponds to pixel-level differential optical depth between the Core and Wings Bands, which can be converted to column density of methan
  • FIG. 4A is another example of gas imaging, showing a natural gas plume emanating from a 10 mm round ball valve orifice inside a 16 mm pipe at a low (household) gas pressure of 1 ⁇ 4 psig in a mild crosswind, outdoors using artificial illumination. This low-pressure release of natural gas is dominated by the buoyancy of methane in air, and accelerates upwards under gravity as a buoyant turbulent plume.
  • FIG. 4B shows a pair of momentum dominated methane jets driven out of a loose threaded hammer union by high internal pressure of 500 psig; they form turbulent gas jets a short distance from slit-like orifices.
  • FIG. 4C A final example of gas imaging is shown in FIG. 4C , where natural gas is leaking from an underground pipe in municipal gas infrastructure in Boston, Mass. By the time the gas percolates up through the soil, it is approximately the same temperature as the ground itself.
  • the prototype system can image the gas emissions from the surface in sunlight as shown, or alternatively using artificial illumination (possibly mixed with sunlight) from above reflecting off the ground, which is absorbed as it passes through the gas twice.
  • 4C illustrates the patchy nature of ground surface emissions, with gas emerging from manholes, storm gratings, cracks in road asphalt and concrete sidewalks, as well as along the side of the road where the asphalt meets dirt and grass. All of these surface emissions may be due to a single leak in a pipe at the bottom of the hill near the end of the street.
  • the spatial distribution of surface leak patches can be useful in bounding the actual leak location in the underground pipe.
  • SWIR imaging sensors for hydrocarbon imaging are described next.
  • semiconductor materials that can be used to fabricate the basic photo-detector sensitive to the SWIR spectrum of light from approximately 1.0 to 2.6 microns, with a dark-current that can be suitably reduced by thermo-electric cooling.
  • semiconductor materials that can be used to fabricate the basic photo-detector sensitive to the SWIR spectrum of light from approximately 1.0 to 2.6 microns, with a dark-current that can be suitably reduced by thermo-electric cooling.
  • extended-response indium gallium arsenide extended-response indium gallium arsenide (extended-InGaAs) commonly grown on an indium phosphide (InP) lattice-mismatched substrate
  • type-II quantum wells made from alternating layers of InGaAs and gallium arsenide antiminide (GaAsSb) grown on an InP lattice-matched substrate.
  • extended-InGaAs photo-detectors are only available as discrete photo-detectors and one-dimensional arrays but not as two-dimensional arrays, while type-II InGaAs/GaAsSb photo-detectors have been successfully fabricated and demonstrated as two-dimensional arrays.
  • MCT Mercury cadmium telluride
  • FIGS. 5A and 5B illustrate the use of one-dimensional SWIR photo-detector arrays in combination with two spectral filters called F-A and F-B, which can be used to create the Core Band and Wings Band filters for methane detection or other hydrocarbons of interest.
  • F-A and F-B two spectral filters
  • a one-dimensional (i.e., linear) 2.5 um-SWIR InGaAs array with 512 detectors is used in the functional prototype methane gas imager.
  • 5A shows a single linear array of photo-detectors with its read-out integrated circuit (ROIC) together with a pair of filters in a frame that is designed to overlay the photo-detector array and alternate between the filters F-A and F-B positioned in front of the detector array.
  • the photo-detector array and its ROIC are mounted on a small thermo-electric cooler and enclosed inside a hermetically sealed package with a transparent window located above the photo-detectors.
  • the alternating filter assembly is positioned outside the package so that each filter overlays the window as the filters alternate in position.
  • This configuration uses a mechanical means to move the respective filters into place at a sufficiently fast rate to support the desired imaging requirements.
  • Other means of alternating spectrally separated bands of light onto a linear detector array are also possible.
  • the prototype gas imager operates at 20 frames/second.
  • FIG. 5B shows another configuration of one-dimensional SWIR photo-detector arrays and filters, where two separate linear arrays with their own ROICs are configured in parallel layout on a common thermo-electric cooler inside a hermetically sealed package with a window located above the pair of photo-detector arrays.
  • Filters F-A and F-B are mounted either in a frame or glued directly to the window, each filter being fixed in place and located above one of the photo-detector arrays.
  • This configuration eliminates the need to mechanically move the filters rapidly and lends itself to higher frame rates.
  • This configuration of two parallel linear arrays of photo-detectors can also be used with an alternating or otherwise changeable filter array such that a new pair of filters is moved into place to overlay the detector arrays.
  • a four-band imager would be created from a dual-linear detector array with alternating pairs of filters in a quad-filter frame, and could, for example, support separate detection and quantification of methane and volatile organic compounds or methane and carbon dioxide.
  • FIG. 6A illustrates the use of a two-dimensional SWIR photo-detector array and ROIC, where an array of four filters, F-A, F-B, F-C, and F-D are configured as stripes that overlay the detector array.
  • the filter stripes can extend across most of the array, with each stripe covering one or more rows of detectors.
  • the detector array and ROIC is to be mounted on a thermo-electric cooler and enclosed in a hermetically sealed package with a transparent window over the detector array.
  • the filter stripes can be configured into an array as a mosaic of individual filters in a frame, or fabricated as a monolithic array, and it is clear that more than four different filters can comprise the array.
  • Two-dimensional 2.5 um-SWIR type-II InGaAs/GaAsSb imaging arrays of size 320 ⁇ 256 pixels are now commercially available. This configuration can be viewed as a collection of many linear arrays covered by a set of spectral filters.
  • FIG. 6B shows a configuration of four discrete SWIR photo-detectors, PD 1 , PD 2 , PD 3 , and PD 4 , arranged in a 2 ⁇ 2 array, each with its own analog read-out circuit and (possibly shared) analog-to-digital converter, and each covered with a separate spectral filter island.
  • the four discrete photo-detectors are to be mounted on a common thermo-electric cooler and enclosed in a hermetically sealed package (e.g., a TO-8 “transistor-outline” metal can) with a transparent window.
  • a hermetically sealed package e.g., a TO-8 “transistor-outline” metal can
  • the spectral filters can be assembled from discrete filters into a spectral filter mosaic, or fabricated as a monolithic array of filter islands, and located outside the window aligned with the photo-detectors below. With the appropriate lens, this configuration forms the equivalent of a single multi-spectral SWIR pixel.
  • This configuration can clearly be extended to more or fewer discrete photo-detectors, each with its own spectral filter. A minimum of two spectrally filtered photo-detectors is required to construct a scanner that can image and quantify gas emissions.
  • This same type of spectral filter mosaic can also be combined with the two-dimensional photo-detector array shown in FIG. 6A , whereby each filter island of the mosaic overlays a small two-dimensional sub-array of even smaller pixels. Upon read-out of the entire detector array, each sub-array of pixels corresponding to the same filter island can be combined into a macro-pixel. This configuration trades off reduced spatial resolution for increased signal in a two-dimensional array of very small photo-de
  • All of the multi-spectral SWIR detector configurations described and shown in FIGS. 5 and 6 utilize additional scanning and focusing optics in order to create two-dimensional spectral imagery from which a gas detection imager can be created.
  • all the detector embodiments shown in FIGS. 5 and 6 lend themselves to packaging in hand-held systems, and can also be configured to operate on moving platforms such as ground vehicles, airborne rotorcraft and fixed-wing platforms, ships, rotating mast-mounted systems, and translating rail-mounted systems.
  • FIG. 7A illustrates a first system diagram for the video gas imaging sensor system.
  • SWIR SWIR
  • the SWIR camera consists of one of the SWIR photo-detector arrays (linear, dual-linear, or two-dimensional) as shown in FIGS. 5A, 5B and 6A , together with its corresponding read-out circuitry and video timing circuitry.
  • This SWIR camera has a SWIR lens (L) that is transmissive to at least the spectral range spanning the wavelengths of interest to sense the hydrocarbon features, approximately 1.0 through 2.6 microns.
  • the spectral filter array positioner Positioned between the SWIR lens (L) and the SWIR camera (SWIR) is the spectral filter array positioner (F) which may include a motor and/or mechanical fixture to properly locate the correct filter(s) in front of the photo-detector array(s) during the exposure of each frame.
  • This combination of SWIR detector array plus filter array corresponds to the various embodiments as shown in FIGS. 5A, 5B, and 6A .
  • the SWIR imaging sub-system also includes a scanning mirror (SM) which sweeps the scene across the spectrally filtered photo-detector array so as to create a two-dimensional field-of-regard.
  • SM scanning mirror
  • the scanning mirror (SM) is typically a one-dimensional scanner that sweeps in a directional perpendicular to the orientation of the filters positioned over one-dimensional detector arrays or the stripes over the two-dimensional detector array.
  • An electronic driver (D) controls the scanning mirror (SM). Synchronization between the scanning mirror (SM), filter positioner (F), and SWIR camera (SWIR) is provided by a micro-controller (C). Two-dimensional image assembly is performed on a micro-processor (P 1 ).
  • FIG. 7B illustrates a second system diagram for the scan gas imaging sensor system.
  • the discrete photo-detectors and spectral filter mosaic (SFM) of FIG. 6B form a single multispectral pixel by means of a defocusing lens (L), and this sensor is scanned across the scene in two directions by mounting it atop a high-accuracy pan-tilt unit (PTU) controlled by micro-controller (C 2 ).
  • PTU pan-tilt unit
  • C 2 micro-controller
  • Two-dimensional imagery is created by raster scanning across a desired, and possibly variable, field-of-regard.
  • Two-dimensional scanning can be accomplished in a compact configuration, for example, by a pair of scanning mirrors or a pair of rotating prisms.
  • Two-dimensional imaging can also be achieved using a single scanning mirror combined with physical movement of the imaging sensor (e.g., translation or rotation in a direction perpendicular to the scan mirror motion) such as by mounting the sensor upon a moving platform (e.g., truck-mounted, airborne, rail-mounted) or rotating the sensor in a mast-mounted configuration.
  • Each imaging sensor system of FIGS. 7A and 7B may also include one or more visible color (RGB) or black and white cameras, laser range finder (LRF) to measure distance from the sensor to a detected leak, global positioning system (GPS) sensor to determine sensor location (and indirectly leak location), inertial measurement unit (IMU) to sense linear and rotational accelerations including direction of gravity, magnetic sensor (Mag) to sense the earth's magnetic field acting as a compass, and/or weather sensors (Wx) to relay local atmospheric conditions including wind speed and direction, all of which is packaged together with one or more processors (P 1 , P 2 ).
  • processors P 1 , P 2
  • processors P 1 , P 2
  • a beam splitter (BS) is incorporated that is preferably dichroic, such that the incident light along the line-of-sight (LOS) is mostly transmitted through the beam splitter for visible wavelengths 0.4 through 0.7 microns, and mostly reflected for SWIR wavelengths 1.0 through 2.6 microns.
  • the reflected SWIR light is subsequently reflected by the scanning mirror (SM) towards the SWIR lens (L) that focuses this light onto the SWIR camera (SWIR) behind the spectral filter assembly (F).
  • SM scanning mirror
  • SWIR lens L
  • F spectral filter assembly
  • the measured range to each SWIR sample can be used to correct the parallax offset between that SWIR sample and its corresponding location in the visible RGB image, using the known spacing of the SWIR, RGB, and LRF sensors.
  • one processor (P 1 ) is associated with the multispectral SWIR camera and is responsible for real-time or near real-time processing of the SWIR multispectral data to create the gas absorption imagery.
  • a separate processor (P 2 ) has a path for accepting the visible camera (RGB) imagery and triggers the other low-bandwidth sensors (LRF, GPS, IMU, Mag).
  • This processor (P 2 ) also communicates wirelessly (or wired) with an external weather sensor (Wx) and a graphical user interface (GUI) implemented on a touch-screen tablet computer.
  • Wx external weather sensor
  • GUI graphical user interface
  • the tablet provides wireless access to an Ethernet or data cloud (E/C), which in turn can be accessed by a remote personal computer (PC).
  • E/C Ethernet or data cloud
  • an artificial illuminator (Lum), as explicitly shown in FIG. 7B is controlled by a micro-controller (C 2 ) and incorporated to enable gas imaging in the absence of sufficient sunlight or for indoor locations.
  • C 2 micro-controller
  • the various sensor embodiments described above can be operated in many different modes.
  • the data gathered from the sensor is analyzed by a processor and used for automatic analysis and decisions (such as triggering of an alarm signal or different operating mode, because a certain limit of gas detection is exceeded) by the processor without being displayed in real-time or near real-time on a display.
  • an image of the received data can be shown on a display (for example for monitoring by a human operator) however no real-time analysis like gas quantification is performed.
  • a third mode an image is displayed and automatic gas quantification is performed, and significant results are automatically stored or sent to remote locations.
  • Other combinations and modes of operation are possible as well, for example in conjunction with the use of low-bandwidth sensors like range and weather sensors.
  • FIGS. 8A and 8B illustrate two alternative imaging geometries of a potential gas leak, shown as a gas jet exiting a hole in a pressurized pipe and expanding into an ambient atmosphere. Illumination provided by the sun is shown in FIG. 8A to transmit directly through the gas jet and ambient atmosphere towards the SWIR imaging sensor, i.e., the sun is roughly in front of the sensor and the gas leak.
  • FIG. 8B artificial illumination comes from near the SWIR imaging sensor behind the gas leak, passes through the gas jet and ambient atmosphere, reflects off background material and heads back to the sensor while passing through the gas jet and ambient atmosphere a second time.
  • a hybrid of these imaging geometries is where the sun is out in front of the sensor beyond the gas leak, but first reflects off the ground then up through the gas towards the sensor.
  • These various imaging geometries differ in three ways; the number of passes through the gas jet (once vs. twice), the optical path length through the ambient atmosphere to be considered (L J vs. 2L R ), and change in spectral illumination of the source S ⁇ due to the reflectivity of the background R ⁇ .
  • the spectral response of the SWIR sensor will be affected by the solar (or artificial) SWIR illumination, reflectivity of the background, absorption by the gas jet, absorption by the ambient atmosphere, the quantum efficiency of the photo-detector array Q ⁇ , and transmission of the filters F ⁇ in combination with the SWIR lens.
  • the geometry of the gas jet is indicative of a momentum-dominated jet forced from an orifice of effective diameter D o under pressure. If the internal pipe pressure is approximately twice the external atmospheric pressure, the jet will exhibit critical flow (also termed “choked flow”) at the orifice where it just reaches the local speed of sound. The internal pressure and temperature determines the density of the gas at the exit, and hence, the mass-flow of gas out the hole. Beyond the exit hole the gas expands rapidly and adiabatically, and beyond an initial zone of complex acoustic waves, the resulting slip flow of the gas relative to the ambient air goes unstable and transitions to turbulence.
  • This turbulence penetrates across the entire gas jet, entraining air into the jet from the sides, which causes the gas to dilute and the jet to expand in a predictable self-similar flow that is invariant to scale.
  • the gas exiting the hole thus shares its initial momentum with the entrained air, thereby losing initial momentum while buoyancy acts to add momentum in the direction of gravity.
  • the buoyancy force acts upwards as the methane at atmospheric pressure is less dense than air.
  • the heavier hydrocarbons will gain downward momentum due to negative buoyancy.
  • the orientation of the pipe and location of the hole will affect the angle of the jet relative to gravity, and the presence of crosswinds will cause the jet to bend with the wind.
  • the invention provides both imagery of the gas leak and quantification in terms of gas present in the jet and mass-flow out the hole along with estimates of hole shape and size.
  • FIG. 9A illustrates an absorption image of a methane gas jet exiting a 1 mm diameter round orifice with an internal pressure of 1300 psig (pounds per square inch—psi “gauge”, i.e., relative to external atmospheric pressure of approximately 14.5 psi).
  • the absorption image is colored according to a pixel-level differential optical depth scale shown to the right. This corresponds to the degree of absorption in the Core Band relative to the Wings Band for the filters used in the functional prototype of FIG. 3A .
  • This pixel-level differential optical depth is directly proportional to the number of methane molecules along each cone of rays between the light source and the photo-detector corresponding to each pixel; this is the so-called pixel column density of the gas.
  • the turbulent structure of the jet is apparent near the top of the jet image. It is clear from the absorption image that the jet diameter grows linearly along the jet axis, as is consistent with the theoretical self-similar solution for turbulent jets. In this image, it is the noise level of the background differential optical depth that determines the boundary of the jet and so limits the visible diameter.
  • FIG. 9B shows cross-sectional profiles of the jet absorption image.
  • the graphs plot differential optical depth vs. pixel number across a row of 512 pixels corresponding to the horizontal lines labeled a, b, c in FIG. 9A . It is apparent from these plots that the diameter of these absorption profiles is increasing along the jet axis, and that the turbulence creates fluctuations in absorption through the jet.
  • the general shape of these plots is entirely consistent with the path length through a cross-section of a round jet in combination with a radial concentration profile of Gaussian shape. Superposed on this smooth theoretical profile are fluctuations in concentration due to turbulence.
  • the maximum of the absorption on each profile should occur on axis of the jet, if the imaging line-of-sight is perpendicular to the jet axis, as this is where the path length through the jet is a maximum and the gas concentration is largest.
  • the gas concentration on axis will decrease linearly along the jet as it expands, while the diameter increases linearly along the axis, and so the product of axial gas concentration with diameter should remain a constant, suggesting the column density along the jet axis should remain constant.
  • these profiles change over time, and so individual pixel values fluctuate.
  • the gas at the front of a jet slice flows slower than the gas at the rear of the jet slice, causing mass to build up between slices of constant thickness.
  • the mass of gas in slices increases linearly along the jet axis, so should the absorption due to that mass.
  • the integrated differential optical depth across each cross-section of the jet image should increase linearly along the jet.
  • the jet width in the absorption image should increase linearly along the jet, where the jet boundary is determined by the noise in the background image. Integrating the absorption across jet cross-sections acts to smooth out the effect of turbulent fluctuations on gas concentration in the jet.
  • FIGS. 10A and 10B plot the automatically extracted jet width and corresponding integrated differential optical depth (integrated-dOD), respectively, along the axial distance (approximately the image row number) for the jet image in FIG. 9A . It is apparent that both quantities follow clear linear trends, and so a least-squares regression line is fit to each quantity. Forming the ratio of integrated differential optical depth to jet width yields an average differential optical depth (average-dOD) value at each axial location along the jet. This ratio is plotted in FIG. 10C , to which a least-squares regression line is fit (starting away from the orifice to exclude the complex acoustic region just outside the hole). It is apparent from FIG. 10C that the slope of this regression line is very small, and that the intercept of the regression line then corresponds to the average differential optical depth extrapolated back to the effective orifice from which the gas leaks under pressure.
  • FIG. 11 plots the integrated differential optical depth (integrated-dOD) along the axis of a natural gas jet emanating from a narrow (50 micron) slit orifice that is 1 cm long, meant to emulate a crack (instead of a hole) in a pressurized line at 60 psig.
  • integrated-dOD integrated differential optical depth
  • the data points are consistent with a power-law behavior of pressure, for which the scaling constant and exponent values are shown on the graph. This is expected since the absorption by the methane gas at the effective exit hole (extrapolating back from the linear boundaries of the jet) will be proportional to the product of the effective orifice diameter and the local gas density, while the gas density is proportional to a power-law of the pressure through the adiabatic equation of state using the ratio of heat capacities for methane. Further experiments will determine the general utility of this specific power-law relationship across a range of orifice diameters and (approximately round) shapes.
  • FIG. 12B plots the measured methane mass flow per orifice area (in grams/sec, divided by orifice area) against internal pressure for numerous experiments using round and slot orifices of different sizes. It is clear they follow the expected linear relationship, with a slope determined by the data.
  • the mass flow out of the orifice is proportional to the product of the area of the orifice and the methane gas density in the pipe (which is proportional to the pressure in the pipe).
  • the Avg-dOD intercept curve scales linearly with effective diameter of a round orifice (as implied by FIG. 12A )
  • the mass flow scales like the square of the effective diameter of a round orifice (as implied by FIG. 12B ).
  • FIG. 13A shows a test setup for imaging and estimating methane mass flow exiting from small round orifices of various sizes at a range of internal pressures up to 1000 psig. Experiments were conducted outdoors in natural sunlight under varying crosswinds.
  • Spectral imagery is taken through at least two filters with transmission exceeding about 5% over wavelength regions that cover the 2350 nm methane feature complex.
  • One filter is narrow (bandwidth approximately 200 nm) and centered at about 2350 nm; call this the Core Filter with transmission F C ( ⁇ ) and integrated transmission F C .
  • the other filter is broad (bandwidth approximately 400 nm), transmitting between approximately 2100-2500 nm; call this the Surround Filter with transmission F S ( ⁇ ) and integrated transmission F S .
  • I W I S - [ F SC F C ] ⁇ I C ( Eq . ⁇ 1 )
  • the gain G CW using in-scene reflector materials (i.e., background materials).
  • in-scene reflector materials i.e., background materials.
  • Eq. 3b a pair of Core and Wings Band images of the in-scene reflector materials (concrete, wood, asphalt, dirt, grass, etc.) together with Eq. 3b to determine an adaptive gain G CW for each reflecting material. It is also possible to generate a library of these gain values for a variety of background materials, and have the user select from a menu the appropriate gain value, or have the sensor system automatically select the appropriate gain value to use while conducting a leak inspection. For direct transmission of sunlight through gas, as in FIG. 8A , the new gain value is obtained by simply imaging in a direction without a background and ignoring atmospheric absorption, using Eq. 3b.
  • I C ( g ) I W ( g ) G CW ⁇ exp - ⁇ [ ⁇ ( g ) - ⁇ ( a ) ] ⁇ 2 ⁇ ⁇ D J + [ ⁇ ( a ) ] ⁇ 2 ⁇ ⁇ L R ⁇ ( Eq . ⁇ 5 )
  • the factor of 1 ⁇ 2 in equation (7b) comes from the double path length through the gas due to reflection of incident light from near or behind the sensor, off the background surface, and back to the sensor. In the case of single pass transmission (e.g., sunlight ahead of the gas leak, passing directly through the gas to the sensor), this factor is simply dropped.
  • Mass CH 4 [ dOD jet ⁇ C - ⁇ W ] ⁇ m CH 4 ( Eq . ⁇ 9 )
  • a surface patch by definition is isolated, surrounded by ambient clear air, with winds that are assumed steady in direction and speed V (the case of gusting winds will be considered below). Gas emerges from the ground, diffuses into the air above, and rises (methane) or falls/lingers (for heavier hydrocarbons) due to buoyancy forces. The wind convects the gas downwind as it continues to disperse and rise (as is typically the case for a natural gas leak from an underground pipe).
  • the mass of methane associated with a surface patch is estimated from spectral imaging, which provides the differential spectral optical depth of methane over the entire patch. Summing the pixels over the entire patch, similar to Eq. 8 for the gas jet, and convert to methane mass over the patch as in Eq. 9.
  • the wind speed V and direction near ground/surface level and assume it is representative of the wind at the emitting surface patch. Also measure range from the sensor to the surface patch, so that pixel dimensions of the patch can be converted to linear dimensions.
  • the steady wind V (cm/sec) blows methane across the patch and away, as it diffuses out of the ground into the air above the patch, and an equilibrium is established in which the surface emission mass flux Q m (grams/sec) is balanced by the windblown mass crossing the downwind boundary of the patch.
  • the methane layer above the surface patch has a characteristic thickness D and concentration c which give rise to the measured differential optical depth dOD at each pixel.
  • the spatial extent of an emitting patch is defined. Construct the bounding rectangle around that patch such that one axis of the rectangle aligns with the wind direction, as illustrated in FIG. 14B . Using the range measured to the patch, convert pixel dimensions of this bounding rectangle to linear dimensions L and W. The volume flux (cm 3 /sec) across the downwind boundary of the patch is equivalent to the volume flux DWV across side W of the rectangle.
  • the methane mass flux Q m grams/sec
  • Q m c ⁇ CH 4 DWV (Eq. 12a)
  • the measured differential optical depth should be scaled by cosine ( ⁇ ) so as to relate to the physical thickness of the layer as denoted by D.
  • ⁇ cosine
  • the embodiments as described above consist of both multispectral SWIR sensors for imaging, detecting and localizing methane and other hydrocarbon gases, and methods to estimate the leak rate or mass flux.
  • Multiple embodiments of sensor systems have been described to enable imaging of gas leaks, and multiple methods have been disclosed for estimating methane mass flux from holes in pressurized lines, and from surface patch emissions due to underground gas pipe leaks.
  • Example imagery and leak rate estimates across a wide variety of conditions illustrate the viability of the sensors and methods.
  • the invention can be used for detecting and quantifying other gases, liquids, emulsions, powders, and solids, in addition to the ones cited above and discussed in detail.
  • multiple spectral filters can be selected to detect ammonia gas, which is both combustible and toxic.
  • fertilizers can be detected and quantified, as can soil wetness and general plant health, thus other embodiments may be well suited for agricultural assessments.

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